As with all things, MT Missiles are something that requires moderation. If
you use 1 or 2 you might was well just aim them off the table. If you take
them in hoards they'll wipe out a fleet if the oposing fleet isn't design
to take them out.
Consider a simple purpose built Cruiser.
Cost: 270
TMF: 80
MD: 4
Hull: 24
FC: 1
MT Missiles: 15
15 Missiles require 30 PDS to take out.
Each missile (if a standard unit) causes on average 7 points of damage.
That's 105 for the salvo.
Now in a game of 1500 points, you can take 5 of those, so 75 missiles
total, requiring 150 PDS to stop, causing 525 points of damage.